Magnetic ink character recognition (MICR) characters are used in a specialized, machine readable type font, used to encode documents such as checks, to facilitate the automated reading of data from those documents using high speed reader/sorters. Other applications employing MICR characters include sales promotion coupons, credit cards, passenger tickets, insurance premium receipts, billing, etc. MICR characters are printed using a magnetic ink or toner to enable reading of the characters by optical and/or magnetic read heads. The term "magnetic ink" means ink or toner capable of being magnetized and sensed. MICR characters are printed using a variety of printing technologies including letterpress, lithography and non-impact electronic printing technology. In order to ensure compatibility between printed MICR characters and devices designed to read these characters, specifications for the geometric shape of the characters and the magnetic wave form generated when the characters are read with a magnetic read head have been developed by the American National Standards Institute (ANSI) and the American Bankers Association (ABA).
To comply with the ANSI and ABA specifications it is necessary that the printed MICR font produce characters which are designed to fall within the geometric shape boundaries (nominal shape plus allowable dimensional tolerances) defined in the specifications. It is further necessary to ensure that the magnetic signal characteristics are maintained within specification by, controlling the magnetic characteristics of the toner used to create the MICR characters and by controlling the amount of toner laid down by the non-impact printing process. Finally, the MICR characters are required to exist within a clear band. The clear band is a horizontal band 0.625 inch (15.9 mm) wide that must be free of any magnetic ink, except MICR, and can be vertically located as determined by the application involved. This clear band, which includes the MICR printed characters, is printed at the bottom of printed checks as specified in the ANSI specifications. A check or multiple checks may be located vertically on the paper as determined by the application involved when printed.
A popular non-impact printing technology employed for the automatic printing of documents, including checks, is electrophotography. This is the printing process often used in photocopiers and laser printers. Electrophotography is an imaging process which has a well known tendency to drift, in terms of print quality, due to a variety of factors including materials aging, sensitivity to variations in ambient relative humidity, batch to batch differences in toners and photoconductors, sensitivity to toner sump fill level, among others. The drift in performance of non-impact printers is typically manifested in undesirable variations in optical density of the printed output. In non-impact printers which are used to print MICR documents, this drift can result in unacceptable variability in magnetic signal level of MICR characters.
In order to control output density variability of printed pages, some presently available copiers and laser printers employ internal control systems which monitor optical density and modulate electrophotographic process variables to control output density within acceptable limits. One variable monitored in some printers is the relative proportions of toner and carrier in the development system. Typically, these control systems are limited to incorporation in higher priced copiers and printers. Furthermore, these systems are intended to control optical image density. Many laser printers, especially low speed devices, do not even employ the optical density control systems found in products such as the Xerox 9790 printer. An attempt is typically made in these systems to run critical subsystems such as development at a "saturated" condition. These open loop approaches cannot compensate for many sources of optical density variability such as those induced by changes in relative humidity, machine aging and batch-to-batch variations in toner properties.
None of the systems described above attempt to, or are capable of, controlling magnetic characteristics of printed images. It is this area of magnetic signal characteristics of images which are important in processing documents such as checks and to which this invention is directed.
It is important to realize that control of optical density of printed images is not sufficient to ensure that the magnetic characteristics of the printed images are being adequately controlled. In the example of the Xerox 9790 high speed MICR laser printer (a 120 page per minute printer) the printing process employs a system to control the optical density of the images as printed. This system is not sufficient to ensure that magnetic signal characteristics of MICR documents are printed correctly. Customers of Xerox 9790 printers typically must utilize expensive test equipment and trained operators to examine sample documents for magnetic signal levels. If tested documents do not have correct signal levels then adjustments are made to the printer by the operator or by a service technician.